1. Field of the Invention
The present invention concerns composite antennas transmitting and receiving simultaneously on two frequencies. A duplexer is generally used to separate the receive channel from the transmit channel, the power in which is much higher. It is necessary to provide a duplexer for each active element of the antenna with the result that the number of duplexers is equal to the number of active elements.
2. Description of the Prior Art
The duplexers are generally larger and heavier than the radiator elements; because of this the use of duplexers leads to a considerable increase in weight and overall dimensions, which is particularly troublesome in space applications. For this reason it is desirable to reduce the weight and the overall dimensions of the duplexer or even to eliminate it altogether.
One solution is to use opposite polarization for transmission and reception, the polarizer carrying out the necessary separation; however, this solution is generally not acceptable in complex systems.
Another solution described in the article by T SHIOKAWA et al in the journal IECE of Japan, technical report, AP 86-60, proposes the use of duplexing radiator elements to achieve operation with circular polarization. The radiator elements proposed in this document provide between 20 and 30 dB of isolation between transmission and reception and it is currently necessary to provide bandpass filters to achieve the necessary separation.
In both cases described the radiator elements are made up of two elementary radiator elements, one operating in transmission and the other in reception. These are ingeniously mounted to occupy the same area. In an implementation the first radiator element is a duplexing dual patch in the form of two plates of dielectric material coated with metal. This solution uses frequency selectivity between the top and bottom patches; it is of interest but it complicates the structure of the radiator element and consequently increases the weight and overall dimensions.
The impedance, the cross polarization performance and the configuration are affected by the disymmetry of the feed system. Also, major redesign is necessary to make the single patch dual and duplexing.
Another solution is described in French patent No. 2 570 546; this is a multifilar helix antenna comprising distinct helical radiators wound on a common core, offset in the angular direction and in a regular way relative to each other, at least two radiators of said antenna being connected continuously to a separate transmitter or receiver device.
This latter solution is not frequency selective and entails a loss of gain.
An object of the present invention is an antenna of the aforementioned type which makes it possible to avoid the problems that have just been described and to eliminate the duplexers. Also, the invention proposes to deal with the problem of passive intermodulation products, which can be a decisive factor in avoiding the use of two separate arrays for Tx-Rx operation (transmission and reception).
The invention has also the capability to cancel on-axis cross polarization and to yield symmetrical radiation patterns.